1. Field of the Invention
The present invention relates to an equalizer and an equalization method, in particular relating to an equalizer and an equalization method for performing equalization of a received signal by calculating an equalization weight based on the minimum means-square error method (MMSE) or the zero-forcing method, using a transmission channel response vector based on transmission channel estimation in time domain.
2. Description of the Related Art
In the wireless communication schemes for next-generation mobile communications, it is important to realize high-speed data transmission. However, as the data rate becomes higher, interference between symbols due to multipaths, namely, multipath interference (MPI) becomes troublesome. There are various methods for suppressing MPI, and use of a linear equalizer is a relatively simple method. A frequency equalizer that performs this equalization process in frequency domain is disclosed, for example by D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency Domain Equalization for Single-Carrier Broadband Wireless Systems”, IEEE Commun. Mag., vol. 40, no. 4, pp. 58-66, April 2002 (literature 1). Also, a plural path samples method is disclosed by M. Matsumoto, S. Yoshida, A Ushirokawa, “Accurate Channel Separation Schemes in MMSE-based Chip Equalizer for HSDPA Mobile Terminals”, in 2005IEICE (The Institute of Electronics, Information and communication engineers) General Conference, B5-120 (literature 2).
FIG. 1 shows one configurational example of a conventional equalizer in which the plural path samples method according to literature 2 is applied to the frequency equalizer described in the literature 1. The conventional equalizer is composed of receiving antenna 1, path-timing detector 2, detection path transmission channel estimator 3, neighbor path transmission channel estimator 4, transmission channel response vector generator 5, serial/parallel (S/P) converters 6 and 10, fast Fourier transform (FFT) portions 7 and 11, weight calculator 8, guard interval (GI) remover 9, equalization filter 12, inverse fast Fourier transform (IFFT) portion 13 and parallel/serial (P/S) converter 14. In the equalizer to which a plural path samples method is applied, representing each path with plural transmission channel estimation samples improves the equalization characteristics under an environment in which plural paths close to each other exist and provides the feature of having tolerance to path-timing errors.
Receiving antenna 1 receives a digitally modulated single carrier signal. Path-timing detector 2 receives as its input samples of the received signal in oversampled number NOS and detects the timings of plural paths using pilot signals included in the received signal. Path-timing detector 2 uses a method of detecting the timings of plural paths having high levels based on the sliding correlation-detected result between the pilot signals contained in the received signal and a known pilot signal sequence, or the like. Detection path transmission channel estimator 3 receives as its input the received signal that is sampled in oversampled number NOS and the path timing detected at path-timing detector 2 and estimates a transmission channel estimation at the timing of the detection path, using the pilot signal contained in the received signal. Neighbor path transmission channel estimator 4 receives as its input the received signal that is sampled in oversampled number NOS and the path timings detected at path-timing detector 2 and estimates transmission channel estimations at plural timings (neighbor path timings) before and after the timing of the detection path, using the pilot signals contained in the received signal. Transmission channel response vector generator 5 receives as its input the transmission channel estimations estimated by detection path transmission channel estimator 3 and neighbor path transmission channel estimator 4 to generate a transmission channel response vector. FIG. 2 is a diagram showing how a transmission channel response vector is generated in transmission channel response vector generator 5. The solid line indicates a transmission channel estimation at a detection path timing while the broken line indicates a transmission channel estimation at a neighbor path timing. The transmission channel estimations (transmission channel estimations sampled for plural paths) at the detection path timing and at the neighbor path timings are arranged to generate a transmission channel response vector. S/P converter 6 performs S/P conversion of the transmission channel response vector generated at transmission channel response vector generator 5. FFT portion 7 receives as its input the transmission channel response vector that was converted at S/P converter 6 and outputs a frequency domain-converted transmission channel estimation. Weight calculator 8 receives as its input the frequency-domain transmission channel estimation output from FFT portion 7, and calculates the equalization weight of the equalization filter based on the minimum means-square error method (MMSE) or the zero-forcing method, etc. GI remover 9 receives the received signal sampled in oversampled number NOS and removes the part corresponding to GI from the received signal. S/P converter 10 performs S/P conversion of the received signal with GI removed at GI remover 9. FFT portion 11 receives the received signal that was converted by S/P converter 10 and performs conversion into frequency domain. Equalization filter 12 receives the equalization weight calculated at weight calculator 8 and the received signal that has been frequency converted at FFT portion 11 and performs equalization of the received signal in the frequency domain. IFFT portion 13 receives the frequency-domain equalized signal output from equalization filter 12 and converts it into time-domain, using IFFT. P/S converter 14 performs P/S conversion of the signal that has been converted in time-domain and outputs a demodulated signal.
In the conventional equalizer, the received signal is equalized and MPI is suppressed in equalization filter 12, but detection path transmission channel estimator 3, neighbor path transmission estimator 4 and transmission channel response vector generator 5 are affected by MPI when the transmission channel response vector used for equalization weight calculation is generated. Consequently, the accuracy of the equalization weight degrades, hence the conventional configuration suffers from the problem that the equalization performance of the received signal is poor.